Particle beam devices in the form of electron beam devices, in particular scanning electron microscopes, have long been known. These are used to examine surfaces of an object (sample). To do so, in the case of a scanning electron microscope, an electron beam (hereinafter referred to as the primary electron beam) is generated by particle source and focused via an objective lens on the object which is to be examined and is situated on a specimen carrier with the help of which the position of the object is adjustable. Using a deflecting device, the primary electron beam is guided in a grid pattern over the surface of the object to be examined. The electrons of the primary electron beam then interact with the object. As a result of the interaction, electrons, in particular, are emitted from the object surface (so-called secondary electrons) or electrons of the primary electron beam are scattered back (so-called backscatter electrons). The backscatter electrons have an energy in the range of 50 eV up to the energy of the electrons of the primary electron beam on the object, whereas the secondary electrons have an energy of less than 50 eV. Secondary electrons and backscatter electrons form the secondary beam, as it is referred to below, and are detected by a detector. The detector signal generated in this way is used to generate the image.
Electron beam devices have a high positional resolution, which is achieved by a very small diameter of the primary electron beam in the plane of the object. The resolution is better, the closer the object is to the objective lens of the electron beam device. It is also particularly important to focus the primary electron beam exactly on the object. It is therefore necessary to accurately determine the position of the object and thus also the distance from the object to the objective lens.
As mentioned above briefly it is also typical for many electron beam devices to use specimen carriers to hold the sample to be examined, so that the position of the sample in the electron beam device is adjustable with the help of the specimen carrier. The position is adjustable in an X-Y plane perpendicular to the beam axis of the particular electron beam device and also in the Z direction corresponding to the beam axis of the particular electron beam device. Some specimen carriers provide tilting of the sample with respect to the beam axis so that the beam axis of the electron beam device is not perpendicular to the surface of the sample. However, it is desirable in many tests of flat samples, e.g., wafers, to have the surface of the sample always perpendicular to the beam axis.
U.S. Pat. No. 4,978,856 describes an autofocus system for a scanning electron microscope having an objective lens for focusing the electron beam on an object. An excitation current supplied to the objective lens is modulated with a periodic signal. Furthermore, the excitation current is wobbled as a function of a wobble signal. The particles emitted and/or backscattered from the sample to be examined are detected. The resulting signal is integrated, thus ascertaining numerous measurement peaks having a certain value. By an approximation method, a value is determined and used to calculate the excitation current of the objective lens for focusing the electron beam on the sample. This calculated excitation current is supplied to the objective lens. In the known method and with the known device, however, it is impossible to determine the position or distance of the sample.
U.S. Pat. No. 5,216,235 describes an autofocus system for a scanning electron microscope which is provided with an optical system for determining the distance of a sample from the objective lens in addition to a particle beam column which has a particle source and an objective lens. The optical system includes a laser whose beams are directed at the sample. However, this known autofocus system is very complex due to the additional optical system. Furthermore, the sample to be examined must have good optical properties to permit an accurate determination of the distance. However, good optical properties are not ensured for all samples to be examined. Diffraction effects may also occur due to structures on the sample, leading to measuring errors.
Accordingly, it would be desirable to provide a method and a device for determining the distance between a sample and a reference point, functioning independently of the type of sample, whereby the method is easy to perform and the device also has a simple design.